Currently, small displacement internal combustion engines for portable power tools such as lawn mowers, compressors and portable generators are not required to meet either state or federally mandated engine exhaust emissions standards as are automobile engines. Many of these small engines are found on handheld power tools. Usually, they are less than 60 bhp and have at most four cylinders, and typically only one or two cylinders. Unlike automotive engines, power tool engines are frequently run for extended durations at full throttle and idle.
The expected phase-in of stringent emissions standards for these small engines poses a difficult design problem. Unlike automobile engines, power tool engines are utilized to power less costly devices and the additional cost of adding an automobile type emissions control system would be prohibitive. Some early attempts at controlling automobile engine emissions utilized carburetors. However, the carburetors became extremely complex and gasoline fuel injection systems were developed instead. An automotive fuel injection system typically utilizes a computerized control system to regulate engine and exhaust emissions while monitoring engine parameters and emissions with electronic sensors.
It would not be feasible to apply modern automobile engine emission control systems to small engines because of the complexity and added cost. Additionally, no single automotive carburetion system would achieve the expected emissions requirements for power tool engines, and none would be capable of being tailored and adjusted for each of the various applications a particular small engine might require. With forthcoming emissions requirements being phased in for small engines, a need has arisen to design a small engine emissions control and fuel delivery system which meets expected standards, yet minimizes the complexity of emissions components and minimizes costs in a manner which will only marginally increase the overall cost of an engine using these devices. Since small engines are generally utilized on moderately priced items such as lawn mowers, chain saws, weed wackers, and generators, the cost of the emissions control system cannot significantly increase the current engine price or else the cost of the device to the consumer will rise to an unacceptable, and possibly non-competitive level in the marketplace, and the consumer will be forced to find an alternative.